Two finger tap on any word to bring up a dictionary / thesaurus:

Once you have that you can't live without it.
Screenshot: ⌘ option 3 takes a screenshot of the whole screen
⌘ option 4 to drag a selection box (press the spacebar to select just a window)
Open QuickTime Player to make a screen recording: it's in the File menu
AirDrop
Keep an icon in the Dock: when you launch an app that isn't in the Dock drag its icon away from the Dock and right back in again. It will stay there.
Use full screen mode for the apps that support it. Gives you a bit more room to work with. You get used to it.
Hide the Dock - same idea
Safari - show the Develop menu in the menubar. Then you can select whatever user agent you want. A website stupidly demands Windows Exploder before it loads? No problem.
Screen sharing: You can share a screen or even control another computer on your LAN. If was sleeping, the other computer will wake up. You can even control another user's account on the remote computer, even if that user is not logged in. How cool is that?
iChat remote screen sharing for when Grandma can't remember how to do something on her Mac
I've only been using these things for 27 years and they keep adding new features, so I still discover new ones.